Seating the crown: cement or bond?

In 1978, dentistry was a significantly less complex calling for me than it is today. If you wanted a filling, I had amalgam.

Oct 1st, 2011

For more on this topic, go to www.dentaleconomics.com and search using the following key words: crown, cement, bond, filling, amalgam, resins, inlay, onlay, Dr. Gary M. DeWood.

In 1978, dentistry was a significantly less complex calling for me than it is today. If you wanted a filling, I had amalgam. I also had gold foil, a great material but a tough technique and pretty expensive. If you did not want gold or silver in a front tooth, I had Adaptic and Concise self-curing composite resins introduced in the 1960s. I also had a newer “light-cured” material, Nuva Fil, that I could form and shape before “setting” it.

If I made an inlay or an onlay, it was gold and cemented with zinc phosphate cement. Crowns were gold or porcelain-fused to an oxidized metal and likewise cemented. All operative and restorative dentistry shared one inescapable requirement — mechanical connection to the tooth is what held it in place. This was the state of the art during my dental education. Yes. I am old. But even as I entered practice, change in operative and restorative dentistry was well on its way to attaining light speed.

Oskar Haggar created an acrylic resin that would stick to teeth in 1949. But until Michael Buonocore made the acrylic white in 1955, and first described an acid etch technique for enamel that improved the stick between tooth and acrylic, it had very little practical application.

While Blaise Pascal’s name was not yet used as frequently in dentistry as it is today, we knew even in 1976 that “stick” is a relative word. While we believed that we were “bonding” to enamel, the fact remains that most of the many failures of bonded restorations in that era were caused by leakage. This bonding stuff had to get a lot stickier before it could become a regular part of everyday dentistry. Thankfully, it did.

Today, bonding is an integral part of almost everything we do, and it has changed for the better many of the restorative procedures that dentistry provides. So the question that really needs to be asked is, “Are bonded crowns better than cemented crowns?”

When a crown is cemented, it is luted to the tooth. Luting is defined as packing or sealing a joint in order to make it tight. The connection is purely physical, not chemical.

That being said, several of the cements used in dentistry do have a degree of stickiness that holds them on to the tooth and restoration. But this feature is not why the cement holds the crown in place. The “bond” strength of luting materials in the literature is rarely reported at or above the generally accepted threshold required of a chemically bonded material. The numbers vary depending on the cement and the material luted, but it is not the strength of the chemistry that holds the restoration in place. It is resistance and retention form combined with the compressive strength of the luting material. The level of chemical attachment is essentially moot.

When the strength of the restorative material is derived from its chemical attachment to the tooth, as it is with leucite-reinforced and feldspathic materials, bonding is required. In my experience, crowns fabricated with cast metal substructures, as well as those made with lithium disilicate and zirconia — which have the inherent strength to endure the rigors of function — do not require bonding to increase the predictability of success.

Success assumes appropriate preparation design (resistance and retention form) for the material selected. The exception to this is bruxers with significant pathway and end-to-end patterns of wear. Every small gain in helping them resist removal of the restoration is generally worth the effort. Thankfully, they represent a relatively small group of the restorations placed each year. In fact, millions of cemented restorations have already exceeded the needs of their users and now lie in an eternal resting place with them.

Every crown does not need to be bonded, even if it can be. Fluoride release, lower solubility, and greater latitude in technique all work in favor of using a dental cement to seat that crown. For teeth and restorations in which these issues are not a concern, there is only one reason not to bond — it is easier to remove a cemented restoration than a bonded one.

While I wish for my patients a life long enough to require replacement of my dentistry, at this point in my career I am hopeful that when removal becomes necessary, it will be a problem another dentist will be dealing with. If that other dentist turns out to be you, I apologize now and hope that better removal instruments have become available. Did I mention that I’m old?

Is a bonded crown better than a cemented one? When the principles of excellent restorative dentistry are applied, the answer to that question will depend on matters other than attachment to the tooth.

Gary M. DeWood, DDS, MS, earned a DDS from Case Western Reserve University in Cleveland, Ohio, and an MS in biomedical sciences from the University of Toledo College of Medicine. He serves as executive vice president for curriculum for Spear Education, teaching and practicing in Scottsdale, Ariz. Contact him at gdewood@speareducation.com.